How do I tell contamination from healthy mycelium?

Most contaminants look very different than mushroom mycelium. The color of Psilocybe cubensis mycelium is white, usually a very clean bright white.
Many molds look similar to mycelium at first, but their color changes to different colors in a matter of days depending on the contaminant species, ranging from green, blue, yellow and red, over to brown and black.
Bacterial contamination is usually colorless, but makes the substrate look wet, slimy, and mycelium will grow very slow and thick on bacteria contaminated substrate, if at all. Bacterial contamination is also detectable by a strong odour emerging the jar, varying from the smell of stinky feet over spoiled milk to really disgusting sweet reek.
While bacterial contamination usually results from unsuficient sterilisation, molds' spores are also often transfered to the substrate at the time of inoculation, either by inoculum itself, the tools used or through the air.